Reversing hot rolling mills which are called steckel mills are used as hot strip mills of relatively low equipment cost chiefly for hot-rolling slabs of stainless steel, special steel, etc.
With reference to FIG. 1 schematically showing the construction of the mill, temperature-holding furnaces 2, 2 arranged at opposite sides of a rolling stand 1 each have a coiler drum 3 in the interior thereof.
The holding furnace is maintained at a temperature, for example, of about 900 to about 1100.degree. C. for holding the material to be rolled at a predetermined rolling temperature. The slab (about 100 to about 200 mm in thickness) is roughly rolled and thereafter processed to a thickness of about 15 mm to about 20 mm. The slab as roughly rolled is passed between rolling rolls 4, 4 of the steckel mill and rolled into a steel strip having a thickness of about 2 to about 10 mm while being alternately wound up on and unwound from the opposed coiler drums 3, 3 in repetition. Incidentally, the left coiler drum in the drawing rotates in a direction A (counterclockwise) when winding up the slab and in the opposite direction B (clockwise) when unwinding the slab, while the right coiler drum rotates in a direction A (clockwise) when winding up the slab and in the opposite direction B (counterclockwise) when unwinding the slab.
The coiler drum is a large hollow cylinder having an outside diameter of at least about 1000 mm, a length of about 2000 to about 3000 mm and a great wall thickness of more than about 50 mm. The drum is conventionally prepared from a 0.35C-24Cr-14Ni-1.4Nb--Fe heat-resistant alloy.
The coiler drum is not only exposed to a high temperature of about 900 to about 1100.degree. C. but also brought into pressing contact over the surface of its trunk portion with an oxide film formed on the surface of the steel strip when winding up the steel strip. While winding up the steel strip repeatedly, therefore, the drum deteriorates over its surface to become rough-surfaced. If the drum is rough-surfaced, the surface spallation is transferred to the surface of the steel strip wound up on the drum, seriously degrading the surface of the steel strip as a product.
To give the coiler drum increased resistance to surface spallation is indispensable to the satisfactory surface quality of the steel strip and to improved durability and diminished maintenance of the drum.
Furthermore, the trunk portion of the coiler drum is repeatedly subjected to a great tightening force due to the winding of the steel strip in a high-temperature environment. Consequently, the coiler drum is susceptible to fatigue fracture due to deformation or hexagonal cracks, which produces an adverse influence on the durability of the drum or on the quality of the product, i.e., the steel strip.
In view of the above problems, the present invention provides a heat-resistant cast steel which is improved in resistance to surface spallation and in mechanical characteristics and which is useful, for example, as a material for forming coiler drums.